1. FIELD OF THE INVENTION
The invention relates to a coil system for the inductive measurement of the velocity of movement of a body which is magnetized at least in one region. The magnetization generally produces an axis of magnetization which is oriented essentially parallel to a specified axis. The coil system comprises a plurality of electrical coils, which are spatially separated from one another in the vicinity of magnetized regions of the body and at some distance from the body. This coil system generates induced voltages proportional to the velocity of movement of the body which voltages are measured or sensed by appropriate electrical circuitry.
2. DESCRIPTION OF THE PRIOR ART
Coil or winding systems are used especially for the lateral stabilization of rotors with magnetic bearings in gas friction vacuum meters, as described in the Journal of Vacuum Science and Technology, Volume 9, page 108 (1972), (1); Vacuum, Volume 32, page 685, 1982, (2), and also for the measurement of the rotation frequency of such rotors, as described in References (1) and (2) and in Proc. 7th International Vacuum Congress, Vienna 1977, Volume 1, page 157, (3). These documents are incorporated herein by reference. The above-mentioned prior art rotors are magnetized essentially parallel to the axis of rotation. The use of coil systems, however, is not limited to the measurement of the movement velocity of rotating bodies. The measurement of velocities of translation movements of a magnetized non-rotating body is also known and have been shown in U.S. Pat. No. 3,470,399 entitled "Linear Motor Velocity Detection Apparatus" which is incorporated herein by reference. The concept "axis of rotation" or "axis of magnetization" are used only to determine a reference direction for the magnetization and location of the coil. Swiss Patent Publication No. 631,812 discloses a coil system for measuring the speed of a print element.
A disadvantage of the coil systems described in the above-mentioned publications (1) and (3) is that they respond not only to the movement of the rotor which is desired to be measured, but simultaneously to other movements thereof, whereby fundamentally interfering noise signals can be superimposed upon the desired measurement signal. In many cases, the installation of compensation coils is necessary to obtain a usable measurement signal. Thus, for example, for the measurement of lateral rotor movements along a movement coordinate perpendicular to the axis of rotation in the rotor with magnetic bearings described in (1), a total of four coils are provided, only two of which are used to compensate for the voltages induced by movements of a magnet located near the rotor. The spatial orientation and sizing of these compensation coils must be tuned very precisely to the specific location of the rotor and magnet, so that the required compensation is adequate. Such a coil combination, however, on account of its special tuning to the interfering magnet, is not capable of compensating for induced voltages which are caused by other sources of interference. For example, the compensation of alternating field induced voltages, such as those which customarily occur in the vicinity of electrically-driven vacuum pumps, makes at least one additional compensation coil necessary in the case described here. Even this coil system, which would then consist of five coils, would still be sensitive to movements of the magnetized rotor in the direction of the axis of rotation, and the rotation frequency of the rotor around the axis of rotation would also produce superimposed interference voltages, unless corresponding additional compensation coils were added, or other measures were taken to suppress the interfering signals. The difficulties described above in the configuration of a coil system which is resistant to interference refer only to the measurement of induced voltages by movements in a movement coordinate. A corresponding effort at compensation would have to be made with known coil systems for the measurement of each coordinate of movement.